Sympathetic nervous system (SNS) prepares the body for action in a situation of acute (immediate) stress. The sympathetic nervous system increases the heart rate and blood pressure this is because the sympathetic nervous system causes the Adrenal Medulla to release Adrenaline into the bloodstream. To return the body back to normal once the stressor has passed,the Parasympathetic nervous system restores the body to it's resting state by slowing down the heart beat and reducing blood pressure.

This response can also be commonly known as the 'Fight or Flight' response. The Sympathetic nervous system also leads to metabolic changes such as mobilisation of fat and glucose into the bloodstream. The Adrenaline released into the bloodstream increases the supply of oxygen to the brain and suppresses non-emergency bodily processes.

This is activated under chronic (long-term) stress. The Hypothalamus leads to a release of Corticotrophin-releasing factor (CRF) into the bloodstream. The Corticotrophin-releasing factor causes the pituitary gland to release Adrenocorticotrophic hormone which is then transported to the Adrenal cortex. The Adrenal cortex once activated releases the hormone Cortisol. The hormone Cortisol can have some positive effects on the body such as a lower sensitivity to pain but can also have negative effects on the body such as a lowered immune response this is most likely due to the gradual shrinking of the Thymus gland (this produces T-cells.)

The hypothalamus is activated when stressors are perceived by higher brain centres. Adrenocortitrophic hormone is transported via the bloodstream but the prolonged (lengthly) release of Adrenocortitrophic hormone causes the adrenal cortex to increase in size to cope with the increased cortisol production.